A standard of care, or best practice, is a guideline for the appropriate treatment of a condition, as established by formal or informal consensus among experts on that condition. Basically, the standard of care for the treatment of a disease is whatever most physicians agree is the best way to treat that disease.

Where multiple treatments for a condition exist, guidelines and medical practice often distinguish first-line therapies from second-line therapies (and sometimes third- or more). The first-line therapy for a given disease is the one that most physicians will prescribe to previously untreated patients as their first treatment, usually due to an assessment of high effectiveness and low comparative risk. If that therapy is ineffective, physicians will move on to a second-line therapy. Because different treatments for the same condition often employ different mechanisms, interact differently with other medications, and cause different side effects, second- or third-line therapies can sometimes benefit patients when first-line therapies cannot. This is one of the reasons it can be helpful to have multiple drugs intended to treat the same disease.

In the context of a clinical trial, a patient receiving the standard of care will be treated with the same drugs they would receive for their condition were they being treated outside of a clinical trial. In this way, researchers can measure the impact of an experimental drug against that of existing treatment.